If a ring laser is rotated, the cavity round trip time becomes different for the two oppositely traveling waves (ODTW). This implies the two oppositely traveling waves have to assume different frequencies. If portions of each of the ODTW are allowed to exit the cavity and are mixed, a beat frequency can be detected that is proportional to the applied rotation rate. This is the ideal laser gyro. In practice, however, there is a coupling of the ORTW due to backscattering of one of the ODTW into the other. At low rotation rates, this coupling causes the ODTW to assume the same frequency and the beat frequency disappears. This frequency synchronization of the ODTW is termed lock-in.
A good ring laser for gyro application should not only have a minimal coupling between counterpropagating waves but also have a stable "standing wave" mode of operation when at rest. Homogeneously broadened gain media such as in solid state and dye lasers have therefore been ruled out for gyro operation.
D. Kuhlke and R. Horak, Opt. Quant. Elect. II, 485 (1979) showed for instance that, in the case of cw dye lasers with weak backscattering coupling, there is generally a strong imbalance between the counterpropagating amplitudes. The laser operation even alternates between the two modes when the backscattering coupling exceeds a certain threshold.